As a sensor capable of performing qualitative/quantitative analysis of an object to be detected with high sensitivity, an SPR sensor utilizing a surface plasmon resonance phenomenon has been proposed and practically used.
As one example of a known structure of such an SPR sensor, there is a structure in which light emitted from a light source is allowed to enter the incidence plane of a high refractive index prism, which has a metal thin film on its plane different from the incident plane, at a predetermined incident angle so that the metal thin film being in contact with an object to be detected is irradiated with the light, and light produced by reflection at the boundary between the prism and the metal thin film is allowed to exit the prism through another plane to detect a change in the intensity of the output light by a detector. In the case of such a structure, light emitted from the light source and then entering the prism produces an evanescent wave at the boundary between the prism and the metal thin film, and on the other hand, a surface plasmon wave is produced at the surface of the metal thin film. The surface plasmon wave is produced (excited) when the incident angle of light or the wavelength of incident light is adjusted so that the wave number of the evanescent wave becomes equal to the wave number of the surface plasmon wave, and at this time the energy of incident light is used to excite the surface plasmon wave and therefore the intensity of reflected light is attenuated. The thickness and refractive index of an object to be detected can be determined from a relationship between the intensity of reflected light and the wavelength at which the intensity of reflected light is attenuated, and the qualitative/quantitative analysis of the object to be detected can be performed using the thus determined refractive index.
In recent years, there has been an increasing demand for an SPR sensor capable of achieving miniaturization and multichannel detection. However, an SPR sensor having the above-described structure using a prism has a problem that its size is large, and even when such an SPR sensor is configured to achieve multichannel detection, its size is further increased. For this reason, it is difficult for such an SPR sensor to achieve both miniaturization and multichannel detection.
On the other hand, an optical waveguide-type SPR sensor using an optical waveguide has been proposed (see, for example, Patent Document 1). Such an optical waveguide-type SPR sensor generally includes an optical waveguide having a core and a cladding surrounding the core and a sensor unit provided by exposing the core through an opening formed in part of the cladding and fixing a metal thin film on at least part of the exposed core so that the metal thin film comes into contact with the exposed core. When such an optical waveguide-type SPR sensor is used to detect an object to be detected, the object to be detected is brought into contact with the sensor unit. Such an optical waveguide-type SPR sensor does not require a prism and therefore can be reduced in size. Further, such an optical waveguide-type SPR sensor can achieve multichannel detection by providing two or more cores (sensor units).
Further, Patent Document 2 proposes an optical waveguide-type SPR sensor including a sensor array unit which is composed of plural waveguides in parallel, each of which is arranged with a metal thin film as a waveguide-type SPR sensor, where the metal films are different in dielectric constant and/or film thickness from each other. Such an optical waveguide-type SPR sensor makes it possible to perform high-resolution and high-sensitive multichannel detection over a wide refractive index range.
Further, Patent Document 3 proposes an optical waveguide-type SPR sensor for multichannel detection including a waveguide unit having two or more cores and a switching unit having two or more switching elements each of which is capable of performing switching between a state where an objected to be measured is detected and a state where an object to be measured is not detected, wherein the switching unit is stacked on the waveguide unit so that the switching elements are arranged along the length direction of the cores. Such an optical waveguide-type SPR sensor can achieve certain results from the viewpoint of miniaturization and multichannel detection. However, such an optical waveguide-type SPR sensor requires two or more switching devices and therefore has a complicated structure, and in addition, it is difficult to produce such an optical waveguide-type SPR sensor at low cost.
The above-described optical waveguide-type SPR sensors can be made smaller in size as compared to an SPR sensor using a prism. However, each of these optical waveguide-type SPR sensors requires an array of two or more optical paths or a switching mechanism to achieve multichannel detection, which limits reduction in size. Further, it is difficult to produce these optical waveguide-type SPR sensors at low cost.